In culturing an animal cell to obtain a natural protein produced by the animal cell, or in culturing an animal cell incorporating a gene encoding a desired protein to produce the desired protein, etc., essential nutrients, such as bases, sugars, amino acids, and vitamins, are added to a culture medium. Further, a mammal-derived extract, concretely, serum such as fetal bovine serum, is usually added in a range of 5 to 20% for proliferation of the animal cell. However, such mammal-derived serum has a number of drawbacks. It accounts for 75 to 95% of the cost for the culture medium, and because of inter-lot differences existing in quality, stable proliferation is not achieved. Moreover, the mammal-derived serum cannot be sterilized in an autoclave or the like, and thus may be contaminated with viruses or mycoplasmas. Although most of these viruses or mycoplasmas are nonpathogenic, they can become additional unknown factors from the viewpoint of stable manufacture. Furthermore, the serum contains more than 500 types of proteins, thus complicating the isolation and purification of the desired protein, the cell product, from the cultured medium. To resolve such problems with stable manufacture, methods using a serum-derived purified protein such as fetuin, insulin or transferrin, instead of serum, are performed. Methods, which use culture medium components extracted from mammals, are also attempted from the viewpoint of production cost.
Recently, however, concern has been expressed over the relation between mammal-derived components and diseases such as mad cow disease, bovine spongiform encephalopathy (BSE), transmissible spongiform encephalopathy (TSE), and Creutzfeld-Jakob disease (CJD). The development of a culture medium for culturing animal cells free from these mammal-derived components has been demanded from the viewpoint of safety.
In culturing an animal cell, the failure to add the above-described mammal-derived components into the culture medium causes a marked drop in the survival rate of cells, and a decrease in viable cell count in the culture broth, at an early stage of culture. These events make long-term culture or large-scale culture impossible.
In order to solve the above-described problem, a method in which an enzymatic degradation product of fish meat or a fish meat extract is added to the culture medium has been reported (Patent Documents 1 and 2). With this method, high production of protein has become possible without fetal bovine serum which was generally believed essential.
However, further improvement has been desired because still higher levels of protein production are preferable from the viewpoint of production cost.
Patent Document 1: International Publication WO 99/63058
Patent Document 2: Japanese Unexamined Patent Publication No. 2003-334068